This invention relates to oscillators that are subject to acceleration effects such as those utilized in Global Positioning Systems (GPS) tracking receivers. The invention particularly relates to the compensation of acceleration induced frequency variations in such oscillators.
Frequency variations in crystal reference oscillators, resulting from resonator sensitivity to acceleration, limit the tracking performance of GPS navigation receivers aboard tactical aircraft. These variations are increased during frequency conversion to L band and generate phase variations at the input to receiver tracking loops. Consequently, an indication of a dynamic condition exists which can erroneously be interpreted as similar line-of-sight dynamic behavior in the GPS signal. More importantly, the resulting phase error generated in the initial measurement unit (IMU) aided tracking loops can become large enough to cause loss of carrier lock. To reduce this possibility, tracking-loop bandwidths are made wider than normally appropriate for good tacking performance in a jamming environment. A requirement exists, therefore, for a crystal reference oscillator that is less sensitive to acceleration.
State-of-the-art approaches to overcomming the problem of g-sensitivity include:
The use of atomic frequency standards;
The use of different crystal cuts in a crystal oscillator; and,
The use of a dual resonator design in a crystal oscillator.
These methods have in most instances been less than satisfactory. Atomic frequency standards tend to be large and expensive and only the effects of constant acceleration are minimized. There is little improvement in g-sensitivity for the ammonia standard currently under development. However, overall improvement in g-sensitivity takes place at the expense of short term frequency stability. The use of doubly rotated crystal cuts improves g-sensitivity by only factors of 2-5. Furthermore, these cuts are more complicated and therefore require tighter manufacturing tolerances. The dual resonator design is only under development at this time, but initially indicates improvement in g-sensitivity by a factor of 10. However, very careful hand matching of crystals are required for whatever performance is achieved.
There currently exists, therefore, the need for an oscillator suitable for use in GPS tracking receivers and other acceleration effected systems that do not have the deficiencies and limitations of presently available devices. The present invention is directed toward satisfying that need.